By looking at the browser screen size stats that we can find online we can see that a large percentage of aspect ratios for desktop sized devices (i.e. those you can't rotate) are widescreen.

I personally struggle to think of many applications outside of watching movies and maybe playing games (I'm not a gamer) where a wide screen aspect ratio is much use.

It's awful for reading any sort of book (hence the Kindle being portrait); not much better for the web (even when only compared to 4:3 ratios); and makes any application with complicated menus trickier due to the loss of vertical screen real estate.

For a TV, it makes sense, but for a computer I don't think I quite get it.

Actually, desktop monitors stand a far better chance of being able to be rotated than laptop screens. The screen doesn't even care, given current OS support, it's the monitor mounting that enables it.
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Marjan VenemaJul 15 '13 at 8:14

You could argue widescreen ratios allow you to process two open windows/documents next to each other, which is more productive than switching between two windows/documents while working on one of them and referencing the other.
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konturSep 12 '13 at 18:02

5 Answers
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I can think of some reasons why today’s larger desktop monitors should have wider aspect ratios than earlier monitors. There’re diminishing returns for screens over 11 inches high.

Given typical heights of seated users and the desks that fit them, making the screen much taller than 11 inches means the user will spend some time reading with the head tilted up, which rapidly becomes uncomfortable. For the best ergonomics, the top of the screen should be level with the user’s eyes. If you were to make a monitor much taller (say, 18”), you’d want to design an unconventional stand that rests the bottom edge of the monitor on the desk and tilts it back more.

At about 11 inches high, users can see a whole sheet of text at 100% zoom, so scrolling is kept to about a page at a time, which works well since users are used to it from physical pages and many documents (e.g., pdfs) are optimized to be read a page at a time.

Given the diminishing returns for heights over 11”, it makes more sense to spend any additional pixels you have to make screens wider to allow the user to display multiple windows at once. It’s far easier to swap between screens with a glance than a click. It’s easier to move the eyes and head side- to-side to switch between two windows than up-and-down, so it’s better for users to display windows side-by-side on a wide monitor than stacked on a tall monitor.

That’s theory. Is that what monitor makers were thinking when they decided to go wide? I bet they were actually thinking about showing movies, like obelia said on this page. Certainly that better explains why so many laptop monitors (including those for super-compact netbooks) are also going for cinematic aspect ratios.

Furthermore, do users understand the potential benefits of large wide desktop monitors, and use them to show multiple windows side-by-side at 100% zoom? I see some do, and some maximize whatever window they’re using, possibly making awkward-to-read super-long lines of text for certain web pages.

Cost.
16:9 displays can be used in both computers and televisions. The larger the volume of display production the cheaper the display. Displays used by both computers and televisions have the highest production volume and lower costs.

In addition to production volume, the screen area for equivalent diagonal screen size measurements are smaller for 16:9 displays. The cost to produce the display is proportional to the area of the display, assuming equivalent dpi. A 20 inch 16:9 display has a display area of 170.52 square inches. A 20 inch 4:3 display has a display area of 192 square inches

Reasons for this transition is alleged to be the productive uses for such monitors, i.e. besides widescreen movie viewing and computer game play, are the word processor display of two standard letter pages side by side, as well as CAD displays of large-size drawings and CAD application menus at the same time.

Even software programmers can utilize the widescreens if they wish to. The IDEs have multiple widgets and panels which require quite a lot of screen estate. correct as a programmer i feel it +1.
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viewPagerSpecialistJul 18 '13 at 6:58